A V-type internal combustion engine includes a cylinder block having first and second cylinder banks arranged in a V-shaped manner. The cylinder block has first and second cylinder heads mounted on the first and second cylinder banks, respectively. Air is drawn into the cylinders of the first cylinder bank via a first intake passage and a first intake manifold. Air is also drawn into the cylinders of the second cylinder bank via a second intake passage and a second intake manifold. In such an internal combustion engine, the arrangement of first and second superchargers in the first and second intake passages to improve output is known in the art. Each supercharger uses the pressure of exhaust gas discharged from the internal combustion engine to pressurize the intake air in the associated intake passage. An intercooler is provided in the portion of the intake passage downstream from each supercharger to cool the pressurized intake air. A throttle valve is arranged in the portion of the intake passage downstream from each intercooler.
Patent publication 1 discloses a variable-nozzle turbocharger as an example of the superchargers mentioned above. The variable-nozzle turbocharger includes a turbine rotated by exhaust gas, a compressor rotated together with the turbine to pressurize air in the intake passage, and a nozzle vane for varying the flow velocity of the exhaust gas delivered to the turbine. The open degree of the nozzle vane is changed to vary the flow velocity of the exhaust gas delivered to the turbine and change the rotation speed of the turbine.
The turbine is rotated at a high speed by the high-temperature exhaust gas. Further, the nozzle vane is designed such that its open degree is variable to adjust the rotational speed of the turbine. As a result, a mechanical abnormality may occur in the turbocharger due to severe engine operation from harsh conditions or wear.
Patent publication 2 discloses an apparatus for detecting such a mechanical abnormality in a turbocharger. The apparatus disclosed in this patent publication is employed in an internal combustion engine having a single intake passage. The apparatus includes a supercharged pressure sensor, which measures intake pressure (supercharged pressure) in the intake passage at the downstream side of a compressor, and a determination circuit, which compare the intake pressure measured by the supercharged pressure sensor with a preset target supercharged pressure. The occurrence of a mechanical abnormality in the turbocharger is determined based on the difference between the measured intake pressure and the target supercharged pressure.
Patent publication 3 discloses a diagnostic apparatus for a turbocharger that is also applied to an internal combustion engine having a single intake passage. This diagnostic apparatus determines that an abnormality has occurred in the turbocharger when the intake pressure in the intake passage at the downstream side of the compressor differs from the target intake pressure and the intake air amount differs from the target intake air amount.
As for an internal combustion engine having two intake passages, any abnormality may occur in the turbocharger of one of the intake passages. In such a case, the normal turbocharger in the other intake passage is feedback controlled so as to compensate for the supercharging deficiency caused by the abnormality. When applying the diagnostic apparatus of patent publications 1 and 2 to this type of internal combustion engine, the difference between the measured intake pressure and the target value is small even if an abnormality occurs in one of the turbochargers. Therefore, it would be difficult to determine an abnormality.
Patent publication 4 discloses a diagnostic apparatus applied to an internal combustion engine including two exhaust passages. The exhaust passages each include a turbocharger turbine. The apparatus of patent publication 4 measures the exhaust pressure in each exhaust passage and determines the occurrence of an abnormality in the turbocharger based on a difference between the measured exhaust pressures.
From the disclosure of patent publication 4, if an internal combustion engine includes a compressor of a turbo charger in each of two intake passages, the intake pressure (supercharged pressure) in each of the intake passages at the downstream side of the compressor may be measured to determine the occurrence of an abnormality in the turbo charger based on the difference between the measured intake pressures.
In an internal combustion engine having two exhaust passages that are completely separated from each other, the apparatus of patent publication 4 enables the difference between the exhaust pressures in the exhaust passages to be obtained with high accuracy. In the same manner, in an internal combustion engine having two intake passages which are completely separated from each other, the difference between the intake pressures in the intake passages is obtained with high accuracy. However, there are internal combustion engines having two intake passages which are connected to each other. In such type of internal combustion engine, there would be no significant difference between the intake pressures of the intake passages. Thus, the occurrence of an abnormality in the turbo charger cannot be accurately determined based on the difference between the intake pressures in the intake passages.
The problems described above are not limited to only turbochargers but may also occur in mechanical superchargers designed to supercharge by utilizing the power of an internal combustion engine.    Patent Publication 1: Japanese Laid-Open Patent Publication No. 2001-107738    Patent Publication 2: Japanese Laid-Open Patent Publication No. 2003-120304    Patent Publication 3: Japanese Laid-Open Patent Publication No. 2002-4872    Patent Publication 4: Japanese National Phase Laid-Open Patent Publication No. 11-509908